Phenolic resins made from phenol (P) and formaldehyde (F) include resoles and novolacs. Resoles have a F/P ratio of greater than 1. As such, resoles have the disadvantage that they contain free formaldehyde. This is in distinction to novolacs which have a F/P ratio of less than 1. As such, novolacs have a deficit of formaldehyde and, therefore, can also serve as formaldehyde scavengers. Novolacs can be used in their cured (or thermoset) state, but they can also be applied in an uncured, thermoplastic state (see WO2007/071387A2 (Dynea Erkner GmbH)).
At ambient conditions, the latter state is typically solid, and they can be described as glassy/congealed or amorphous solid materials. Upon temperature increase, the material consistency becomes softer and beyond the melting range becomes a liquid.
The manufacturing of novolacs is well known to a person skilled in the art, as may be found in A. Knop & L. A. Pilato, Phenolic Resins, Springer Verlag, 1985, Chapters 3 & 5.
Sometimes, it might be preferred to apply a novolac in a liquid form at around room temperature. A first approach (GB 899,776) dissolves novolacs in hydroxides of alkalines or earth alkalines. After drying or curing, the hydroxides remain on the surface, thereby increasing the pH to values, which are often undesirable.
In a second approach, novolacs can be dissolved in liquid resoles. However, as resoles exhibit a kinetically controlled self-curing process, they only have a limited storage stability, which then also limits the storage stability of the novolac-resole solutions.
In yet a further approach, novolacs can be dissolved in an organic solvent, such as described in U.S. Pat. No. 4,124,554, U.S. Pat. No. 5,200,455 or U.S. Pat. No. 4,167,500, disclosing the use of organic solvents to produce aqueous dispersions of novolacs. The use of such solvents is often unwanted, because they are often flammable and/or hazardous for the environment.
Whilst water would not have these disadvantages, novolacs do not dissolve therein and are not miscible therewith.
U.S. Pat. No. 5,670,571 (Georgia Pacific Resins) describes a method to produce an aqueous dispersion of a novolac resin and its use as a binder system for thermal insulation. Hereby, the water is added to the molten novolac and, therefore, this method is limited to novolacs having a melting point below 100° C. The process further employs surfactants (e.g. lecithin) and protective colloids (e.g. casein or polysaccharides sugar, or guar gum). Furthermore, the method of U.S. Pat. No. 5,670,571 produces only novolac particle sizes of 0.1-20 μm.
U.S. Pat. No. 6,130,289 (Lord Corporation) describes an aqueous dispersion of phenolic resins of the resole or novolac type. Dispersed within the aqueous phase is the reaction product of a phenolic resin precursor and a modifying agent wherein the modifying agent includes at least one ionic group and at least one functional moiety that enables the modifying agent to undergo condensation with the phenolic resin precursor. The modifying agent contains at least two distinct functional groups wherein one of the at least two functional groups is an ionic pendant group and another of the at least two functional groups is capable of reacting with a phenolic resin precursor.
Whilst the disclosures of the above documents relate to in-situ formed resins, U.S. Pat. No. 4,124,554 (Union Carbide Corporation) describes post-formed aqueous phenolic resin dispersions, i.e. to the dispersing of resins after these have been produced into particles. This invention uses polyvinyl alcohol (PVOH) to disperse reacted resins. To this end, a water miscible organic coupling solvent is needed in an amount from about 15 to about 30 percent. In contrast, the present invention does not need an organic solvent.
EP 0084681 (Union Carbide Corporation) describes a process for producing particulate novolacs by acid condensation with sulphur-containing catalysts or mixtures of them with following addition of water and neutralization. In order to achieve particulate novolac resin formation, up to 5% of particular protective colloids are added. The particles can be isolated by conventional means, and may result in mean particle sizes of up to 1 mm. Alternatively, the protective colloid can be used to form an aqueous dispersion of small resin particles (max. 50 μm). Suitable protective colloids are polysaccharides, whilst hydrolyzed PVOH or carboxymethyl cellulose (CMC) was reported to be unsuitable. The resins according to EP 0084681 may be cured by curing agents.
The use of starches in the process of encapsulating materials such as foods are described in, e.g., U.S. Pat. No. 4,812,445 (Nat Starch Chem Corporation) and WO99/25207 (Danisco), however, neither of these references suggest encapsulating novolac resins.
Despite the foregoing, there remains a need for a relatively inexpensive stable aqueous composition (such as a dispersion) of particulate novolac resins which can be applied as a film or coating to a substrate (such as fiberglass, agricultural/horticultural products, and lignocellulosic materials which include composite board, plywood, parquet, laminated veneer lumber (LVL), laminated flooring, door, wood for door frame and paper). An objective of the present invention is to fill this need.